feat(delphi): D10 — Clojure-parity rep comment selection (PR 8)

Replaces the pre-D10 botched-port `select_rep_comments_df` with a
single-pass reduce that mirrors Clojure `select-rep-comments`
(math/src/polismath/math/repness.clj:212-281).

Sits on top of PR 14a in the spr stack.

## Helpers added (top-level in `repness.py`)

- `passes_by_test(s)` — Clojure `passes-by-test?` (repness.clj:165-170).
  OR'd on (rat, pat) and (rdt, pdt) z-sig-90. NO `pa >= 0.5` gate; the
  pre-D10 Python gate was an over-restriction with no Clojure analog.

- `beats_best_by_test(s, current_best_z)` — Clojure `beats-best-by-test?`
  (repness.clj:133-139). Strict `>` on `max(rat, rdt)`.

- `beats_best_agr(s, current_best)` — Clojure `beats-best-agr?`
  (repness.clj:142-162). Four-branch agree-priority logic:
    1. na == 0 AND nd == 0 → reject.
    2. current_best AND current_best.ra > 1.0 → compare 4-way signed product
       `ra * rat * pa * pat`.
    3. current_best (else, ra <= 1.0) → compare `pa * pat` only.
    4. No current_best → accept if `z90(pat)` OR `(ra > 1.0 AND pa > 0.5)`.
  `current_best` stores the RAW row (Clojure repness.clj:250) so the
  ra/rat/pa/pat surface stays available across iterations.

- `_finalize_row_for_output(row, *, is_best_agree=False)` — Clojure
  `finalize-cmt-stats` (repness.clj:173-188) + best-agree flagging
  (repness.clj:262-264). Emits `best_agree=True` and `n_agree=na` for the
  best-agree slot.

## `select_rep_comments_df` rewrite

Signature now: `(stats_df, mod_out=None) -> List[Dict[str, Any]]`. Drops
the `agree_count` / `disagree_count` kwargs (Clojure has only a cap of 5).

Per-row state `{sufficient, best, best_agree}` updated by the helpers.
Final assembly: dedup best_agree from sufficient → sort by metric
(agree_metric for repful=='agree', disagree_metric for 'disagree')
→ prepend finalized+flagged best_agree → take 5 → agrees-before-disagrees.

The caller in `conv_repness` drops the `_stats_row_to_dict` wrapping step
(the new function returns finalized dicts directly).

## Two pre-D10 bugs fixed alongside the rewrite

- `pa >= 0.5 / pd >= 0.5` over-gate in the passing filter — removed (no
  Clojure analog).
- "Fill from other category" + "first row" fallback blocks — deleted. The
  `:best` / `:best_agree` mechanism IS the Clojure fallback.

## Tests (18 new in `tests/test_discrepancy_fixes.py`)

- TestD10PassesByTest (4): agree-side, disagree-side, neither, no pa-gate.
- TestD10BeatsBestByTest (3): None-best, max(rat,rdt), strict `>`.
- TestD10BeatsBestAgr (6): one per Clojure branch + boundary.
- TestD10SelectRepCommentsBoundary (5): empty input, single unvoted row
  → best fallback, sufficient-empty-best-agree-only, take-5 cap +
  agrees-before-disagrees, **the eviction edge case** (best_agree outside
  sufficient evicting 5th-highest-metric).

## Eviction edge case — flagged

`take(5)` runs AFTER prepending best_agree. If `best_agree` was kept by
`beats_best_agr` as a non-significant agree-priority fallback (failed
`passes_by_test`, qualified via Branch 4) AND `:sufficient` already has 5
entries, the prepend pushes total to 6 and `take(5)` evicts the
5th-highest-metric sufficient entry — possibly a strong dissenting view.

Mirrors Clojure exactly for blob parity. `# TODO(parity-eviction)` comment
at the take(5) site in the production code; entry added under
"Pending — needs team discussion" in PLAN.md; pinned by the synthetic
test above.

## Re-xfailed with updated reasons (D14 / D1 upstream divergence)

Six per-shared-(gid, tid) blob-comparison tests were previously xfailed as
"D5/D6/D7/D8/D10: no shared comments to compare". After D10 there ARE
shared comments (overlap ~20% on vw cold_start), but the per-(gid, tid)
stats still mismatch because Python and Clojure place different
participants in the "same" group ID. That's upstream PCA/KMeans
group-membership divergence (D14 / D1), not D10. Updated xfail reasons
point at D14 / D1. D10 itself is verified by the 18 synthetic tests.

Affected: TestD9ZScoreThresholds::test_z_values_match_clojure,
TestD5ProportionTest::test_pat_values_match_clojure_blob,
TestD6TwoPropTest::test_rat_values_match_clojure_blob,
TestD7RepnessMetric::test_repness_metric_matches_clojure_blob,
TestD8FinalizeStats::test_repful_matches_clojure_blob,
TestD10RepCommentSelection::test_rep_comments_match_clojure.

## Suite delta

- Pre (post-14a baseline): 295 passed, 12 skipped, 58 xfailed.
- Post:                    313 passed, 12 skipped, 58 xfailed.
- Delta: +18 passed (the 18 new D10 synthetic tests), 0 failed, no new
  xfailed.

## Documentation

- `delphi/docs/PLAN_DISCREPANCY_FIXES.md`: PR 14a row marked landed
  (#2564), PR 8 (D10) marked in-flight, eviction concern added to
  "Pending — needs team discussion".
- `delphi/docs/CLJ-PARITY-FIXES-JOURNAL.md`: "Session: PR 8 — D10 rep
  comment selection (2026-06-11)" entry with full scope, suite delta,
  and decisions log pointer.

## /goal mode

This PR is part of an autonomous run (`/goal`) targeting D10 + D11 + D12 +
golden snapshots as a stacked PR series. Decisions made autonomously
(key naming, return type, etc.) are documented in
`~/polis/D10_D11_D12_GOLDENS_DECISIONS.md` for batch user review at the
end of the run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

commit-id:1b681fef

Author: jucor

delphi/docs/CLJ-PARITY-FIXES-JOURNAL.md

@@ -1347,3 +1347,99 @@ PR 14a unblocks (in stack order):
    readability reference. Research agent produced a clean split proposal:
    `_build_group_comment_index` (plumbing) + `_compute_per_group_stats`
    (math) + 5-line orchestrator.
+
+## Session: PR 8 — D10 rep comment selection (2026-06-11)
+
+Landed in `/goal` mode (autonomous run targeting D10 + D11 + D12 + goldens
+as a stacked PR series). Decisions made without inline user check are
+documented in `~/polis/D10_D11_D12_GOLDENS_DECISIONS.md` for batch review.
+
+### What landed
+
+**Production code (`delphi/polismath/pca_kmeans_rep/repness.py`)** — added
+3 top-level helpers + `_finalize_row_for_output` + rewrote `select_rep_comments_df`:
+
+- `passes_by_test(s) -> bool` — Clojure `passes-by-test?` (repness.clj:165-170).
+  OR'd on `(rat, pat)` and `(rdt, pdt)` z-sig-90. **NO `pa >= 0.5` gate** —
+  the pre-D10 Python gate was a botched-port over-restriction with no
+  Clojure analog.
+- `beats_best_by_test(s, current_best_z) -> bool` — Clojure `beats-best-by-test?`
+  (repness.clj:133-139). Strict `>` on `max(rat, rdt)` vs current best z.
+- `beats_best_agr(s, current_best) -> bool` — Clojure `beats-best-agr?`
+  (repness.clj:142-162). Four-branch agree-priority logic:
+    1. `na == 0 and nd == 0` → reject.
+    2. Current best AND `current_best['ra'] > 1.0` → compare 4-way signed
+       product `ra * rat * pa * pat`.
+    3. Current best (else, `ra <= 1.0`) → compare `pa * pat`.
+    4. No current best → accept if `z90(pat)` OR `(ra > 1.0 AND pa > 0.5)`.
+- `_finalize_row_for_output(row, *, is_best_agree=False)` — Clojure
+  `finalize-cmt-stats` (repness.clj:173-188) + best-agree flagging
+  (repness.clj:262-264). Adds `best_agree=True` and `n_agree=na` keys for
+  the best-agree slot.
+- `select_rep_comments_df(stats_df, mod_out=None) -> List[Dict[str, Any]]` —
+  single-pass reduce over `stats_df.to_dict('records')` mirroring Clojure
+  `select-rep-comments` (repness.clj:212-281). Per-row state
+  `{sufficient, best, best_agree}` updated by the three helpers; final
+  assembly is dedup-best-agree-from-sufficient → sort by metric →
+  prepend best-agree → take 5 → agrees-before-disagrees.
+
+**Caller (`conv_repness`)** — dropped the `_stats_row_to_dict` wrapping
+step since `select_rep_comments_df` now returns finalized dicts directly.
+
+**Two pre-D10 bugs fixed alongside the rewrite** (research-agent flagged):
+- `pa >= 0.5 / pd >= 0.5` over-gate in the passing filter — removed. No
+  Clojure analog; was dropping legitimate candidates.
+- "Fill-from-other-category" + "first-row" fallback blocks — deleted. The
+  `:best` / `:best_agree` mechanism IS the Clojure fallback.
+
+**Tests** — 18 new tests (in `tests/test_discrepancy_fixes.py`):
+- `TestD10PassesByTest` (4 tests): agree-side significant, disagree-side
+  significant, neither significant, no `pa >= 0.5` gate.
+- `TestD10BeatsBestByTest` (3 tests): None-best, max-rat-rdt, strict `>`.
+- `TestD10BeatsBestAgr` (6 tests): Branch 1 (na=nd=0), Branch 2 (ra>1),
+  Branch 3 (ra<=1), Branch 4 z90(pat), Branch 4 (ra>1 AND pa>0.5), Branch 4
+  rejection.
+- `TestD10SelectRepCommentsBoundary` (5 tests): empty input, single unvoted
+  row → best fallback, sufficient-empty-best-agree-only, take-5 cap with
+  agrees-before-disagrees ordering, **the eviction edge case** (best_agree
+  outside sufficient evicting 5th-highest-metric).
+
+**Re-xfailed with updated reasons** (D14 / D1 upstream divergence):
+- `TestD9ZScoreThresholds::test_z_values_match_clojure`
+- `TestD5ProportionTest::test_pat_values_match_clojure_blob`
+- `TestD6TwoPropTest::test_rat_values_match_clojure_blob`
+- `TestD7RepnessMetric::test_repness_metric_matches_clojure_blob`
+- `TestD8FinalizeStats::test_repful_matches_clojure_blob`
+- `TestD10RepCommentSelection::test_rep_comments_match_clojure`
+
+Why xfailed despite D10 landing: D10 enables shared comments in the
+selection (overlap rises from 0% to ~20% on vw cold_start), but
+per-(gid, tid) stats still mismatch because Python and Clojure put
+different participants in the "same" group ID. That's upstream
+PCA/KMeans group-membership divergence (D14 / D1), not D10. D10 is
+verified via the 18 synthetic helper + boundary tests above.
+
+### Suite delta (pre/post D10)
+
+- Pre (post-14a): 295 passed, 12 skipped, 58 xfailed.
+- Post (this PR): 313 passed, 12 skipped, 58 xfailed.
+- Delta: +18 passed, 0 failed, 0 new xfailed. The +18 matches the 18 new
+  D10 synthetic tests exactly.
+
+### Decisions made autonomously (under `/goal` mode)
+
+See `~/polis/D10_D11_D12_GOLDENS_DECISIONS.md`. Highlights:
+- **S1**: Python convention key names (`repful`, `best_agree`, `n_agree`)
+  instead of Clojure hyphens. Math blob alignment is a future PR.
+- **S2**: `select_rep_comments_df` returns `List[Dict[str, Any]]` instead
+  of `pd.DataFrame` — variable extra keys (best_agree flag) make list-of-
+  dicts cleaner than DF-with-NaN-columns.
+- **D10.1**: Two pre-D10 bugs (pa>=0.5 gate, fill-from-other fallback)
+  folded into D10 rather than separate PRs — the rewrite replaces the
+  function so a surgical fix would be more noise than value.
+- **D10.7**: Real-data blob-comparison tests re-xfailed with reasons
+  pointing at D14/D1, not softened to overlap-thresholds — more honest.
+
+### What's Next
+
+PR 9 (D11) on top of D10 in the same spr stack.

delphi/docs/PLAN_DISCREPANCY_FIXES.md

@@ -28,8 +28,8 @@ This plan's "PR N" labels map to actual GitHub PRs as follows:
 | PR 7 (D8) | #2522 | Stack 15/17 | Fix D8: finalize comment stats |
 | PR 12 (D15) | #2523 | Stack 16/17 | Fix D15: moderation handling |
 | (K-inv) | #2524 | Stack 17/17 | Fix K-means k divergence: preserve row order |
-| PR 14a (scalar deletion) | — (in flight) | — | Delete dead scalar paths in `repness.py`; migrate blob injection tests to vectorized |
-| PR 8 (D10) | — (WIP) | — | Fix D10: rep comment selection — **NEEDS REWORK** |
+| PR 14a (scalar deletion) | #2564 | — | Delete dead scalar paths in `repness.py`; migrate blob injection tests to vectorized |
+| PR 8 (D10) | — (in flight) | — | Fix D10: rep comment selection — single-pass reduce matching Clojure |
 | PR 9 (D11) | — (WIP) | — | Fix D11: consensus selection — **NEEDS REWORK** |
 | PR 10 (D3) | — (WIP) | — | Fix D3: k-smoother buffer — **NEEDS REWORK** |
 | PR 11 (D12) | — (WIP) | — | Fix D12: comment priorities — **NEEDS REWORK** |
@@ -915,3 +915,14 @@ Tagging this as a follow-up. No code changes until we discuss.
 - **`to_dynamo_dict` parallel inline implementations** were refactored to
   route through the same helpers as `to_dict` in PR #2523 follow-up. No
   further action needed.
+- **D10 take-5 eviction edge case** (2026-06-11). The Clojure-parity
+  `select_rep_comments_df` introduced in PR 8 mirrors Clojure exactly:
+  `take(5)` runs AFTER prepending the `best_agree` slot. When `best_agree`
+  was kept by `beats_best_agr?` as a non-significant agree-priority fallback
+  (i.e. it failed `passes_by_test?` but qualified via Branch 4) AND
+  `:sufficient` already has 5 entries, the prepend pushes the total to 6
+  and `take(5)` silently evicts the 5th-highest-metric `sufficient` entry
+  — possibly a strong dissenting view. Mirrored for blob parity; flag for
+  future product review. See `# TODO(parity-eviction)` in
+  `delphi/polismath/pca_kmeans_rep/repness.py::select_rep_comments_df` and
+  the synthetic test `TestD10SelectRepCommentsBoundary::test_take_5_eviction_when_best_agree_outside_sufficient`.